In the past, SAW devices were used in military, or special, applications. In recent years, SAW devices have found use in civil applications, such as FM tuners and TV receivers, and have attracted much interest rapidly. SAW devices are manufactured as delay lines, oscillators, filters, etc. The features of these various SAW devices are that they are small in size, light in weight, have high reliability, and can be fabricated by processes similar to those used in the case of integrated circuits. Hence, they can be easily mass-produced. Today, SAW devices are mass-produced as electronic parts indispensable to the industry.
By way of example of the prior art SAW device, a surface acoustic wave (SAW) resonator is now described. This resonator comprises a piezoelectric substrate on which arrays of strip electrodes made from an electrically conductive material are formed. Two similar lattice-like reflectors each consisting of dielectric substance, conductive substance, grooves, and ridges are formed on opposite sides of the electrode arrays. When a voltage of a certain frequency is applied to the electrodes, an electric field is applied to the surface of the piezoelectric substrate in the gaps between the electrodes. Then, a mechanical distortion is produced in proportion to the voltage because of the piezoelectricity of the piezoelectric substrate. The distortion propagates in both directions as surface waves at an acoustic velocity determined by the material of the piezoelectric substrate. The surface waves are reflected by the reflectors on both sides back to the strip electrodes, causing the electrodes to resonate.
Generally, these various SAW devices are hermetically sealed in metallic containers to remove external noise. Taking account of hermetic seal and resistance to errosion, the containers are usually plated with nickel or other metal.
In these SAW devices, conductive foreign matter intruded before the containers are hermetically sealed or matter peeling off from the plating of the containers may adhere to the strip electrodes, causing short circuits between the electrodes. This varies the electric impedance or presents other problems. As a result, the reliability of the SAW devices deteriorates, making it difficult to mass-produce them.
Japanese Patent Laid-Open No. 3957/1984 discloses an invention of a hermetically sealed electronic part package which comprises a circuit substrate, a vibrator installed on the substrate, and a casing mounted over the vibrator. Japanese Utility Model Laid-Open No. 78935/1978 discloses an SAW device comprising a piezoelectric substrate, electrodes formed on the substrate, a solder layer formed around the surface of the substrate on which the electrodes are formed, a sheetlike object having lead wires extending from the electrodes, and another solder layer formed around the sheetlike object. The sheetlike object is placed on, and bonded with adhesive to, the substrate.
In these SAW devices, the surface on which the electrodes are formed is hermetically sealed by the casing or sheetlike object. Therefore, a peripheral circuit for exciting the device must be formed on a separate substrate.
Also, the chips of SAW devices are generally very small. Therefore, where each individual SAW device is packaged, when they are installed on external circuits, a sufficient care must be taken, and the operation is performed with quite poor efficiency.